Mass spectrometry has been used to determine the extent of modification and the specific sites of modification on biomolecules. MS-based approaches have many advantages, including generally rapid analyses without radiolabeling. The MS analysis of protein-DNA crosslinks has been investigated using mass spectrometry. Products and digests have been analyzed by both positive and negative ion MALDI mass spectrometry and LC in combination with electrospray mass spectrometry. Additionally, products following crosslinking have been purified by SDS-PAGE and investigated by mass spectrometry. Free radicals are implicated in oxidative stress and are associated with a wide range of diseases and disorders. In this work, we have investigated the sites of radicals trapped by DMPO on deoxyribonucleosides and proteins using LC/MS/MS.